The present invention relates to on-line spectroscopy systems. More particularly, the present invention relates to a fiber-optic based system for on-line spectral analysis of fluids and other substances.
It is well known that the absorption of light by a fluid or other substance (i.e., sample), as a function of wavelength, forms the basis of spectroscopic analysis. The analysis can take place in a number of spectral ranges, ranging from the ultraviolet and visible range where molecules absorb light due to electronic transitions, to the infra-red range where light absorption corresponds to vibrational transitions. In the near infra-red (NIR) region, absorption corresponds to vibrational transitions in the bonds between hydrogen atoms and the rest of the molecule (referred to as Xxe2x80x94H bonds).
The exact wavelength at which these Xxe2x80x94H bonds adsorb light depends on the structure of the molecule. This forms the basis of NMR analysis, as different molecules, such as aromatics, aliphatics and olefins, have different absorption spectra.
A number of devices have been, and continue to be, employed for spectral analysis of samples. The devices are typically employed to measure the reflection, transmission, fluorescence or the light scattering from xe2x80x9con-linexe2x80x9d samples.
The xe2x80x9con-linexe2x80x9d devices typically comprise three parts: (i) an analyzer, which includes a light or other radiation source and a detection system, (ii) an optical probe for transmitting the light or other radiation to and receiving it from the analyzed sample and (iii) suitable optical fibers for guiding the light or other radiation between the analyzer and the probe. Illustrative is the device manufactured and distributed by the Perkin Elmer Corp.
The Perkin Elmer device includes a probe member (or head) capable of measuring the absolute transmission signal of the sample. The probe includes dual cells, one of which is for sample analysis while the other is a dummy reference cell. The device also includes a mechanical shutter to alternately block and unblock the sample and reference optical paths.
The Perkin Elmer device suffers from a number of disadvantages. First, the probe is made up of many optical components, such as lenses, a beam splitter, prisms, optical windows, and the like, which make it awkward, expensive and difficult to properly align. Second, the probe is inefficient in that at least xc2xe of the signal is lost in the course of the double pass through the beam splitter, which is used to split the beam to the reference and sample optical paths.
A further optic-based system is disclosed in U.S. Pat. No. 5,044,755 (I. Landa, et al.). In this system, the light emerging from a fiber bundle is collimated by a lens. The optical ray is then guided through a sample cell and reflected back to the same lens which focuses the light into the same fiber bundle. Some of the fibers are used to guide the light into the transmission probe while some of the fibers are used to guide the light out to the detection system (i.e., analyzer).
Optic-based systems distributed by UOP Guided Wave Inc. and Galileo Electro-Optics Corp. similarly employ a transmission probe in which the light emerging from the fiber, whether a single fiber or a fiber bundle, is collimated by a lens that guides the light through the sample cell. On emerging from the sample cell, the ray is collected by another lens which focuses the optical ray onto a second output fiber.
Finally, in U.S. Pat. No. 5,442,437 (T. Davidson) a flow cell and optic probe assembly is disclosed wherein one probe directs light into the cell chamber and a second probe collects the emitted light. The lens that is typically employed in conventional systems is thus eliminated.
A major drawback of the noted optic-based systems is that the spacing between the optic probe members (i.e., path length) generally cannot be controlled to a satisfactory level. Since the amount of light absorption (i.e., absorbance) is directly proportional to the path length of light passing through the sample being analyzed, the path length must be closely controlled.
A further drawback of conventional optic-based systems is that they typically exhibit either a path length in the range of 0.1 xcexcm to 1.0 xcexcm or a path length greater than 1.0 mm. Thus, as discussed in detail herein, since absorbance (A) varies with the wavelength of the absorbing material in direct proportion to the path length (i.e., the absortivity and concentration of the absorbing material held constant), it will be appreciated that conventional optic-based systems have limited applicability.
An additional problem associated with conventional optic-based systems is that the probes tend to become clouded or contaminated when positioned in the flow path of the material to be analyzed (i.e., sample flow path). The design of several probes also make them susceptible to erosion and catastrophic failure (e.g., tip fracture) when exposed to the sample flow path for an extended period of time.
It is therefore an object of the present invention to provide a sample flow cell and fiber-optic probe system that overcomes the above-discussed deficiencies with conventional optic-based spectroscopy systems.
It is another object of the present invention to provide a sample flow cell and fiber-optic probe system that positions the probe members directly in the sample flow path with minimal risk of fracture.
It is another object of the present invention to provide an on-line sample flow cell and fiber-optic probe system having means for accurate control of the path length.
It is yet another object of the present invention to provide a sample flow cell and fiber-optic probe system that includes efficient means of minimizing contamination and clouding of the probe members when positioned in the sample flow path.
In accordance with the above objects and those that will be mentioned and will become apparent below, the sample flow cell and probe apparatus in accordance with this invention comprises a flow cell having a flow passage and means for introducing a flowable material into the flow passage for flowing therethrough, the flow cell includes a cell chamber in communication with the flow passage, the cell chamber including a region of reasonable flow, a first probe for transmitting light of a given wavelength into the cell chamber, the first probe including a first optic cable member having first and second ends, the first end being disposed in the region of reasonable flow; a second probe for detecting emission light from the flowable material, the second probe including a second optic cable member having first and second ends, the first end being disposed in the region of reasonable flow; the first and second probes having a path length in the range of approximately 20 xcexcm to 1 mm; and control means in communication with the second ends of the first and second optic cable members for providing the light to the first end of the first optic cable member and analyzing the emission light detected by the first end of the second optic cable member.